1. Field of the Invention
The present invention relates to a method and a device for correlating cylinder charge and intake valve lift in the case of an internal-combustion engine having valves with a variable valve gear. Furthermore, the invention relates to a computer program product for carrying out the method and to an automobile comprising the device.
2. Description of Related Art
In modern engine control systems, a torque-based strategy is commonly employed. The automobile driver or any other torque demand source requires a certain torque. This torque demand is then converted into torque influent control variables, and the control variables are adjusted according to the conversion result in order to cause the engine to deliver the required torque. Typical control variables by which the torque output of an internal-combustion engine can be influenced include the cylinder charge, i.e. the quantity of air loaded into the cylinder, the air-fuel mixture ratio, and the ignition spark angle in the case of a spark ignition engine.
A variation of the cylinder charge can be achieved by means of a throttle flap in a conventional throttled engine, by means of actuation devices changing the intake valve opening angle or opening height, or by increasing the air pressure with a turbo- or other supercharger. In the case of an engine equipped with continuously-variable valve lift (CVVL) technology, the cylinder charge is mainly influenced by the variable maximum intake valve lift, i.e. the maximum height to which the intake valve opens during one engine cycle.
To be able to use the air cylinder charge as a control variable in the case of an engine equipped with continuously-variable valve lift technology, a calculational model correlating the cylinder charge to the maximum intake valve lift is necessary. Such a model is also needed for determining the fuel quantity to be injected in order to achieve a desired air-fuel ratio of the cylinder charge. However, the cylinder charge depends not only by the maximum intake valve lift but in particular also on the intake spread, exhaust spread, rotational engine speed, and the intake conditions, i.e. air temperature and pressure of the air taken in by the engine. Therefore, a calculational model based on a high-dimensional, empirically determined map that takes into account all variables influencing the cylinder charge would require large calculational, in particular memory resources that are impractical or expensive to realize in an on-board control device.
EP 1 030 042 B1 discloses a method of determining the fresh air cylinder charge for unthrottled engines. The calculation is based on a reference two-dimensional map dependent on the maximum intake valve lift and the rotational engine speed, defined at reference cam phasing angles and at reference intake conditions. A first correction factor compensates for other cam phasing angles than the ones corresponding to the reference map. A second correction factor corrects for different intake conditions.
However, when applied to realistic ranges of rotational speed and delivered torque of typical engines, the charge calculation model appears to be not reflecting properly the trapped air mass in the cylinder for all operating points. It is desirable to provide a way of correlating maximum intake valve lift and cylinder charge that yields precise results over a large range of operating points, without recurring to resource-intensive calculation means such as high-dimensional maps.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.